A. Field of the Invention
The present invention relates to a method for making optical devices with predetermined refractive index profiles, e.g., refractive index gradients. More particularly, the invention relates to a method for treating porous preforms composed of vapor-deposited silica glass or the like to modify the refractive index of selected regions thereof.
B. Description of the Prior Art
Vapor deposition of doped silica is the most commonly employed technique for forming optical waveguide fibers. Such fibers are usually doped with GeO.sub.2 or P.sub.2 O.sub.5 to form a glass having a refractive index greater than that of silica, or with B.sub.2 O.sub.3 or fluorine to form a glass having a refractive index less than that of silica. Because of its low absorption at long wavelengths, fluorine is preferred over B.sub.2 O.sub.3 for transmission at wavelengths beyond approximately 1.2 Mm.
Fluorine has been employed as the sole dopant in single-mode fibers having a silica core and a fluorine-doped silica clad. Fluorine has also been added along with other dopants in the core of a single-mode fiber to change the zero dispersion wavelength, and it has been added with other dopants to obtain a desired combination of properties such as refractive index and viscosity. For example, fluorine and P.sub.2 O.sub.5 can be added to silica to form a diffusion barrier having the same refractive index as a silica substrate tube.
Difficulties have been encountered in trying to deposit fluorine directly as a dopant in vapor-deposited glass. U.S. Pat. No. 4,335,934 reports that fluorine tends to reduce the rate of deposition of doped silica glass on the inner surface of a substrate tube. It has been found that the addition of a fluorine-containing compound to the reactant stream emitted by a flame hydrolysis burner tends to decrease the rate of deposition of glass soot collected on the mandrel. Also, seeds were commonly formed in the resultant article when both fluorine and germania were co-deposited with silica.
A further disadvantage has been experienced while attempting to form fluorine-doped silica by supplying C.sub.2 F.sub.6 to a flame hydrolysis burner. Even though the amount of C.sub.2 F.sub.6 is increased, it is difficult to increase the amount of fluorine in the resultant glass to more than about 0.6 wt. %. This may be due to the fact that a fluorine-doped silica particle may not be immediately formed in the burner flame; rather, the fluorine must diffuse into the silica particle as it travels from the burner to the soot preform. Such diffusion must take place within a fraction of a second. The partial pressure of fluorine adjacent to the silica particle is very low since the fluorine supplied to the flame diffuses into the ambient atmosphere. Furthermore, some of fluorine adjacent the silica particle reacts with hydroxyl ions present in The flame to form HF; this fluorine is no longer available to dope the particle.
A more successful approach to the doping of vapor deposited silica or doped silica glass has involved the treatment of the glass with fluorine after deposition as a porous material but prior to consolidation of that material to clear glass. In this process, the porous preform is contacted with gaseous fluorine or a fluoride compound which is then absorbed into the glass as an index-modifying dopant.
In published French patent application No. 2,428,618, an optical fiber having a graded index core and cladding wherein fluorine is the only dopant is produced by depositing porous silica layers on a starting member, heating the porous preform to cause partial sintering but not complete fusion, and then forming an index gradient in the preform by the slow diffusion of fluorine into the preform. Doping profiles attainable by this technique are however limited to diffusion gradients; step profiles and/or control of the slope of the gradient are not feasible.
Published Japanese patent application No. 56-50136 describes a fluorine treatment for use with graded index preforms which is intended to tailor the peripheral index profile of the core preform to improve fiber bandwidth. In that method the preform is not sintered prior to treatment in the fluorine (or boron-containing) atmosphere, but the nature of the fluorine treatment is such that only the edge profile of the core of the preform is adjusted.
In published European patent application No. EP0139532, the fluorine treatment described is applied to the entire cross-section of a preform, effecting a general decrease in refractive index across most or all of the preform diameter. In that process, the preform is first heated to a temperature below sintering for dehydration, and is then further heated in the presence of fluorine at a temperature below that of rapid consolidation to uniformly reduce the refractive index of the preform. If the core material of the preform is supplied as a glass rod or a high density soot layer, the fluorine treatment affects mainly the refractive index of the cladding material.
In each of these prior art methods for doping porous soot preforms with vaporized dopants such as fluorine or boron, control over the exact doping profile is difficult or inexact. Only step profiles and diffusion gradients are described; thus the choice of doping profile is limited and in some cases the attainment of a specific profile may require extra process steps, such as separate drying, doping, and consolidation of the core and cladding elements.